The present invention relates to a solution for etching copper and its method of use to make fine-line copper elements where the etching is precise to close tolerance. The method is particularly suited for articles having complex curvature. The copper elements are commonly used in radomes, which are the immediate focus of this application.
Circuit board elements are often etched on flat, plated surfaces in conventional etching processes using photolithographic techniques to define lines to specific tolerances of about xc2x11.0 mil (i.e., 0.001 in). The initial line width of typically about 2-15 mils is patterned in the photoresist before introducing the etchant. Generally, a strong acid such as ferric chloride is used as the etchant. Ferric chloride penetrates quickly through the copper film, and, because of its vigorous etch rate, only modest tolerances are achievable. The etchant begins to etch laterally (i.e., undercut) as it simultaneously continues to etch into the metal. The initial photoresist pattern must define openings in the photoresist narrower than the desired line width in the metal, because undercutting will occur. The etchant will penetrate beneath the photoresist film. The circuit board etching process must be monitored closely so that etching can be halted at the precise time. With a conventional etching process, like those using ferric chloride, it is difficult, if not impossible, to achieve line widths ranging from 3-10xc2x10.25 mils (0.003-0.010 in) even on flat boards, because of the etch rate and inherent process delays. Our applications require this close tolerance on large parts having complex curvature. Control of line width is more difficult if the film has a complex curvature. With large parts, it becomes more difficult to flush or to neutralize the etchant to stop the etching. Using conventional circuit board etching processes, this precision is lost.
An etching solution especially valuable for precision etching of vapor deposited copper films of complex curvature comprises about 0.5 ft3 etchant/ft2 copper film (with a nominal thickness of about 0.1 mil), the solution being about 5 gm per liter of a mild copper etching acid, such as CuCl2, and at least about 125 gm per liter of NaCl or another suitable salt in deionized water. We reproducibly achieve line widths of about 3-10 milxc2x10.25 mil.
We use this etchant to etch copper to high precision and can reliably and reproducibly prepare etched copper surfaces having complex curvature with elements etched to close tolerances as small as xc2x10.00025 in. We incorporate these copper surfaces into radomes, generally securing each copper surface between two dielectric sheets.